Echo ranging, often referred to as echo location or echo sounding, is well known in the prior art as a non-contact distance measuring method. Ultrasonic systems have found application in liquid and solid particulate level monitoring, flow rate monitoring, camera range-finding and numerous other short-range distance measuring systems. Radio frequency systems have found use in both high resolution and long-range measurement systems.
Echo ranging systems transmit a discrete burst of pulses towards the reflective object the distance of which from the source is to be measured. Following reflection, the echo is received and the elapsed time between transmission of the burst and reception of the echo is noted. Assuming a homogeneous propagation medium, the measured distance is directly proportional to this elapsed time.
Common to all such systems is a circuit for detection of the received echo including a discriminator for differentiating between actual echoes and acoustic and/or electronic noise. Typically this is accomplished by feeding the received signal through one or more stages of amplification and/or filtering to discriminate the desired signal followed by a rectification (detection) stage. The unipolar signal is then compared to a preset noise threshold to indicate the reception of a valid echo.
In many applications, the gain must be time-varied to compensate for transmission losses. While in radar systems, the medium is essentially lossless, so that signal attenuation is solely a function of the fourth power of the distance traversed, in ultrasonic systems the attenuation is a function of both the medium and the wave distribution and the signal is rapidly attenuated. In either case, the attenuation versus the time (distance) function is predictable and can thus be compensated for by increasing the receiver gain with time, according to the inverse function; the amplified signal is then compared to a fixed threshold for discrimination.
Examples of this method can be found in the prior art. One such system employing time varying gain is described in U.S. Pat. No. 4,000,650, issued to Ellery P. Snyder, Mar. 20, 1975. Several disadvantages are unavoidable with such an approach. Generally a DC control voltage is applied to a voltage-variable gain stage to implement the time varying gain function. Such a stage is constructed with the use of a circuit element whose resistance or effective transconductance is varied by the DC control voltage. Presently available elements share common problems of nonlinearity, limited dynamic range and relatively high cost. Additionally most such elements require calibration to offset the wide initial transfer function tolerance.
Another method, described in U.S. Pat. No. 4,145,741, issued to Donald Nappin, Mar. 20, 1979 described a system employing log amplifiers to effectively subtract the log of the attentuation function from that of the received signal. Although some advantages are gained from such a method, the cost and complexity of suitable circuitry are again relatively high.